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Q. Yan, N.R. Perdue, D. Possin; Matricellular Protein SPARC Plays an Important Role in the Maintenance of Lens Epithelial Homeostasis . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4320.
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© ARVO (1962-2015); The Authors (2016-present)
The biological significance of SPARC in ocular lens is underscored by the observations that all SPARC–null mice develop cataracts. An understanding of how SPARC functions in lens epithelial cells will provide insight into how SPARC contributes to the maintenance of lens transparency.
Lens epithelial cells (LEC) were cultured from wt and SPARC–null lenses. The proliferate response of LEC to EGF, IGF, FGF–2 and recombinant (r)SPARC protein was measured by 3H–thymidine incorporation. The cell migration was performed by monolayer wound healing assay and transwell migration assay. In vitro invasive capacities of LEC were evaluated on matrigel–coated transwells. Immunocytochemistry and immunoblotting were used to evaluate the expression of interested proteins in LEC. Morphology of lens epithelial cells was studied by electron microscopy (EM).
Wt and SPARC–null LEC showed similar proliferation rates in response to growth factors (above) by 3H–thymidine incorporation. rSPARC inhibited the incorporation of 3H–thymidine into newly synthesized DNA in the presence of low–serum level in culture medium, but DNA synthesis was increased in serum–free culture medium, suggesting effects of SPARC on LEC proliferation are dependent on serum factors. Although the wt and SPARC–null LEC did not display significant differences in cell migration, the null cells exhibited enhanced invasive ability on Matrigel invasion assay, and expressed higher levels of α–SMA, MMPs, and other ECM modulators compared to wt LEC, indicating lens epithelial cells lacking SPARC could be transdifferentiated into mesenchymal–like cells. In addition, SPARC inhibited TGF–ß mediated fibroblast–like morphology and production of α–SMA in long–term cultured LEC. SPARC alone appeared ineffective in inhibition or activation of Smad2/3 signaling pathway, however, pretreatment of SPARC in LEC significantly inhibited TGF–ß induced phospho–Smad2/3 in the cells. EM examination of LEC showed that cells lacking SPARC developed large vacuoles in the cytosol; dilated endoplasmic reticulum, large numbers of ribosomes, a dark stain of nucleus, and invasive protrusions were observed in SPARC–null LEC.
SPARC plays an important role in the maintenance of lens epithelial integrity and characteristics. SPARC appears to prevent lens epithelial–mesenchymal transdifferentiation through inhibiting TGF–ß mediated phospho–Smad2/3 pathway.
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